Guidelines: add a motion principles page #331

Summary

Add a Motion brand book page: durations, easing curves and a few do's and don'ts for animation, drawn as curves and timing bars, with a reduced-motion note.

Where to look

• docs/guidelines-pages.md and CONTRIBUTING.md, "Adding a Brand Guidelines page"
• lib/guidelines/pages/spacing-layout.ts: scales drawn as bars
• lib/effects/skeleton.ts: how the app already handles reduced motion

Behaviour

• Derive the durations from one base value (for example 150, 250 and 400 ms) and draw each easing curve as an SVG path.

Acceptance criteria

• The page appears in the PDF, the website and the Markdown export, and can be switched off.
• Contents numbering stays correct.
• A unit test checks the curve geometry.

Getting started: fork the repo, then pnpm install and pnpm dev (Node 22+, pnpm 11, no API keys). Before opening a PR, run pnpm typecheck && pnpm test && pnpm lint && pnpm format:check && pnpm check:dashes && pnpm build (CI fails on unformatted files, so pnpm format first). See CONTRIBUTING.md and the first contribution walkthrough.

Everything needed is in the repository. This doesn't need maintainer access, secrets or paid services. Comment below to claim it, and ask questions here any time.